The radiator fan on my 82 4WD wagon doesn't work when the headlights are on. It works when the parking lights are on though. the thermoswitch on the radiator seems to be good as the fan works normally and cools the engine fine with the lights off. is this an alternator issue or something? or a bad ground? I'm planning to replace all the grounding straps anyway so I'll see if that helps.

Not alternator. . Maybe grounding.

Try it w/a jumper cable connected between battery negative and the body of the fan thermoswitch.

Check the bonding wire from top brace of radiator to radiator support.

 

Also, check the body ground wire from battery - cable to body.

When the grounding wire from the engine to the frame gets over 200,000 miles, it gets hot when the engine is running because it has lost it's conductivity.  The hot point is on the frame end of the wire.  The wire will also be hard where it has lost conductivity.  Replace the wire if it has hot spots.

 

Another wire the wears out on my 86 gl10, is the hot wire coming off of the hot battery terminal, that feeds into the bottom of the fusible link breaker box.  The hot point is at the batter terminal end.  Replace half of wire by splaying in new wire to old wire, because standard auto parts store couplers do not work for this jury rigged connection.

 

The above two hidden defects in the electrical system are primarily responsible for accessories not working when the headlights are on, in my 86 gl10.

You just picked this car up and the previous owner wired the fan to come on with a switch and the typically unused parking light was an easy option?

I've seen people make the fans come on with a switch far more often than a switch causing an unrelated system to work/not work. 

 

Of course if you've owned this car awhile and this just started then that's ruled out, looks like previous poster has seen some similar issues.